Electrical Measuring Instruments Questions and Answers

Answer: D

The current coil of an electrodynamometer wattmeter carries the load current, which alternates, and a solid conductor or core near it would carry eddy currents that cause loss and errors. The key gives the lamination as the way to reduce eddy current losses, option (D). Damping is provided by an air vane, creeping error is prevented by a compensating coil or a shading arrangement, and hysteresis loss is a feature of iron cores and is not what the lamination mainly reduces.

Answer: C

In a PMMC instrument a change in temperature changes the stiffness of the control springs, and it would change the torque they give. To neutralise this effect, two springs are used, wound in opposite spiral directions, so that the change of one is compensated by the change of the other. So the effect is neutralised by the opposite spiral arrangement of springs, option (C). A heat sink, a cooling medium and a shading coil are not used for this purpose.

Answer: D

The material for control springs should have high resistance to fatigue, should be non-magnetic so that the field does not affect it, and should have a low temperature coefficient so that its stiffness does not change with temperature. The springs also carry current to the coil, so the material should have a low specific resistance, not a high one. So the property that is undesired is high specific resistance, option (D); the other three are desired properties.

Answer: C

A megger is the instrument preferred to find an earth leakage fault in cables. It is a high-range ohmmeter that applies a high direct voltage and measures the insulation resistance of a cable, so a low reading to earth shows a leakage fault.

A voltmeter, an ammeter and an energy meter measure voltage, current and energy, and they cannot show the insulation resistance of a cable that is not connected to a supply. So the third option is the answer.

Answer: C

Both statements are true, so the answer is 'True, True'. An energy meter should be fixed at a convenient height and, as the first statement says, preferably not below 1 m from the ground, so that it is out of the way of water and damage and can be read easily.

The second statement is also true, because a correct meter must not creep and register when there is no load on the circuit, and the standards limit this creeping. The key gives the third option, and 'False' for either statement is wrong.